Field of the Invention
The present invention relates to a method for producing a pressure-applying fixing roller (hereinafter referred to as a “fixing pressure roller”).
Background Art
A fixing unit of a copying machine or a printer employs a roller, and examples of the roller include a roller made of a metallic (iron, aluminum, etc.) core and an elastic layer (silicone rubber) (hereinafter referred to as fixing roller), and a roller made of a metallic core, an elastic layer, and a release layer (fluororesin tube) (hereinafter referred to as a pressure roller). Generally, the elastic layer of such a fixing pressure roller is made of foamed silicone rubber. Foamed silicone rubber has low hardness and ensures a wide fixation width, and has such a low heat capacity that the heat of a heat source built in a fixing roller or a fixing belt facing the elastic layer is not readily transferred thereto.
There has been proposed a pressure roller having a sponge silicone layer produced by a composition containing a thermally curing-type silicone rubber composition (millable silicone rubber) with a peroxide vulcanizing agent—p-methylbenzoyl peroxide—and a foaming agent—azobisisobutyronitrile; vulcanizing the mixture; and foam-molding the vulcanized product (see Patent Document 1).
Such foamed silicone rubber is generally produced via a chemical foaming process, in which a foaming agent is incorporated in advance into a base rubber, and the foaming agent is gasified through heating. However, various problems arise in molding and vulcanization. More specifically, one problem is that, when primary vulcanization and secondary vulcanization are performed after extrusion of an unvulcanized silicone rubber, large bubbles generate in the vicinity of a metallic core, resulting in interfacial delamination and reduction in rubber strength. This phenomenon is significant, when a long time is provided between extrusion and primary vulcanization. In actual production steps, the leaving time after molding may vary, thereby significantly causing an unstable foaming condition, which is problematic.
Meanwhile, when primary vulcanization is performed immediately after extrusion by means of a vent-type extruder, in which unvulcanized rubber is kneaded and extruded under reduced pressure, no foaming attributed to a chemical foaming agent occurs, which is also problematic.
Under such circumstances, demand has arisen for a technique which allows easy molding to produce foamed silicone rubber.    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2003-345158